The goal of this study is to characterize the molecular mechanisms that detect Ca2+ rises in synaptic terminals and identify how these components drive synaptic communication between neurons using the Drosophila model. Research from my lab and others has demonstrated the importance of the Synaptotagmin (Syt) family of Ca2+ binding proteins in translating synaptic Ca2+ dynamics into the regulation of neuronal communication. Given mutations in Synaptotagmins have been found to cause both CNS and neuromuscular disorders in humans, it is essential to understand how this protein family regulates synaptic communication. Presynaptically, Ca2+ entry triggers synaptic vesicle (SV) fusion through a synchronous phase that occurs within milliseconds, and a slower asynchronous phase that can last for hundreds of milliseconds. Postsynaptic Ca2+ influx also regulates membrane trafficking at synapses to drive retrograde signaling pathways that link neuronal activity and synaptic growth. In Aim 1, we will define how Syt1 interfaces with Complexin and the SNARE complex to control whether SVs fuse synchronously, asynchronously or spontaneously. We will also determine how mutations in human Syt1 and Syt2 dominantly disrupt SV fusion and lead to neurological disease. In Aim 2, we will characterize how Syt7, a popular candidate for the asynchronous and short-term facilitation Ca2+ sensor, regulates SV release. In particular, we will test if Syt7 negatively regulates release by inhibiting fusogenicity of SVs in the reserve pool and activating Ca2+-dependent recycling routes that decrease SV replenishment rate at AZs. In Aim 3, we will analyze how the Syt4 Ca2+ sensor interfaces with postsynaptic SNAREs to mediate retrograde signaling and control synaptic plasticity. In addition, we will identify new components of retrograde signaling using genetic screens to assay for defects in postsynaptic membrane trafficking. In Aim 4, we will perform localization studies and functional analysis of the final four members of the Drosophila Syt family to test their putative roles in DCV fusion and neuropeptide release (Syt-? and Syt-?) or Ca2+-independent regulation of membrane trafficking (Syt12 and Syt14). Using CRISPR technology to mutate and endogenously tag these Syt members, we are in an exciting position to define the function of the entire Syt family within a single organism for the first time. In summary, these experiments will provide a comprehensive picture of how the Synaptotagmin family of proteins control synaptic membrane trafficking to regulate Ca2+-dependent neuronal communication from multiple intracellular compartments.